Some Economics of Sight Evolution

Sight is obviously of high utility in most situations. However, the amount of resources we spend on sight is constrained by our metabolism.

Our brain uses up about 20% of our energy, and sight is an energy intensive part of that.

Nature manages this by providing us with two types of visual neurons. The economics is that they have different features, and nature appears to trade them off.

Brisk neurons capture movement. They can send more data per second, but they send it in smaller packets. This means they potentially need to send information often, with each firing requiring energy.

Sluggish neurons capture capture edges and borders. They send less total information per second, but they send it less often in bigger packets. Less often means less energy.

Even so, both types of neurons could fire 150 to 250 times more often than they do. They don't because they are constrained by the metabolic constraints of the body.

This is the Marshallian aspect. Marshall pioneered the use of partial equilibrium, in which all other things are equal. This is a pedogical technique to assume away broader details that are not relevant to the situation at hand. In the case of individual neurons, the metabolic constraint imposed by the body is exogenous; we don't have to worry about it too much to highlight the tradeoff.

You'd expect the result to be a more sparing use of the brisk neurons, and that is exactly what is found: in guinea pigs (where spotting movement might be critical for survival) only 30% of the visual neurons are brisk.

Genesis

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